You really should use the NewPing library...it allows you to:
- ping very quickly (sez up to 30 Hz)
- set a maximum distance where pings beyond that distance are read as no ping
- eliminates the one second delay from pulseIn
- makes your code cleaner and more compact
- allows you to easily use a median reading (to eliminate the occasional bogus hit)
etc., etc., etc.
Also, how far apart are your sensors and where are they pointing? They may be interfering with each other.